Recently, organic photoreceptors have been mostly used for electrophotographic photoreceptor (hereinafter referred to as a photoreceptor). The organic photoreceptors have more advantages than inorganic photoreceptors in that materials capable of receiving various light from visible light to infrared can be used, materials which are less influenced by environmental contamination can be used, and that inexpensive materials can be used. However, the organic photoreceptors have a disadvantage of poorer mechanical durability than the inorganic ones. In terms of effective use of resources, photoreceptors preferably have sufficient mechanical durability and long lives. Therefore, for the purpose of increasing mechanical durability of the organic photoreceptors, a number of arts forming a protection layer are disclosed. However, it is difficult for the organic photoreceptors to have long lives only by increasing the mechanical durability. Prevention of adherence of foreign particles and improvement of toner transferability are essential therefor to have long lives.
Even photoreceptors having good mechanical durability occasionally produce abnormal images when used for long periods due to paper powders and adherence of toner additives. A part of the photoreceptor these adhere to is not properly charged or irradiated, resulting in production of abnormal images. Photoreceptors having poor mechanical durability can prevent production of abnormal images because outermost surfaces thereof are abraded and new surfaces consecutively appear. However, they are difficult to have long lives. Therefore, it is very important to prevent adherence of foreign particles.
Toners do not waste when having higher transferability. When untransferred toner (toner remaining on a photoreceptor even after transferred onto a paper or an intermediate transferer) increases, a cleaner does not work well, resulting in shorter life of a process cartridge.
It is very important as well to increase transferability of a toner. Prevention of adherence of foreign particles and improvement of toner transferability are referred to as releasability in combination because of representing the same repellency.
A mechanical durability improver and a releasability applicator need to be combined to have both of mechanical durability and releasability, but which is not easy.
A number of arts forming a protection layer on the outermost surface of a photoreceptor and dispersing an inorganic particulate material in the protection layer to improve mechanical durability of the photoreceptor are disclosed.
Japanese published unexamined application No. 2002-139859 discloses an electrophotographic photoreceptor including at least an electroconductive substrate, a photosensitive layer overlying the substrate, and a protection layer including a filler and overlying the photosensitive layer.
Further, a number of methods increasing hardness of the surface of a photoreceptor are disclosed as well. Japanese published unexamined applications Nos. 2001-125286 and 2001-324857 disclose increasing hardness of a protection layer of a photoreceptor to prevent a magnetic particulate material transferred onto the photoreceptor from damaging the photoreceptor when pressed by a transferer or a cleaner thereto when a magnetic brush is used as a charger.
Japanese published unexamined application No. 2003-098708 disclose increasing hardness of a photoreceptor to prevent the surface thereof from being abraded when a blade cleaner is used. Specific means of increasing hardness of the surface of a photoreceptor include including crosslinkable materials such as thermosetting resins and UV curing resins in a protection layer of the photoreceptor. Japanese published unexamined applications Nos. 5-181299, 2002-6526 and 2002-82465 disclose methods of using a thermosetting resin as a binder of a protection layer to improve mechanical durability and damage resistance thereof.
Japanese published unexamined applications Nos. 2000-284514, 2000-284515 and 2001-194813 disclose including a siloxane resin bonded with a charge transportability imparting group in a protection layer to improve mechanical durability and damage resistance thereof.
Japanese patent No. 3194392 discloses a method of forming a charge transport layer using a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond, and a binder resin to improve mechanical durability and damage resistance of the charge transport layer.
Japanese published unexamined application No. 2004-302451 discloses a method of hardening a tri- or more functional radical polymerizable monomer having no charge transport structure and a monofunctional radical polymerizable compound having a charge transport structure to form a charge transport layer.
Further, Japanese published unexamined application No. 2005-99688 discloses a method of hardening a tri- or more functional radical polymerizable monomer having no charge transport structure and a monofunctional radical polymerizable compound having a charge transport structure, and further dispersing a filler to form a protection layer. These methods noticeably improve mechanical durability of a photoreceptor. Particularly, photoreceptors including a curing resin disclosed in Japanese published unexamined applications Nos. 2004-302451 and 2005-99688 have good mechanical durability and damage resistance.
The outermost surface effectively has a lower energy to impart releasability thereto. A low surface energizer may externally coated on or internally included in a layer to low surface energize the surface of a photoreceptor. As an external additive, zinc stearate is typically coated thereon to impart releasability thereto. However, the low surface energizer deteriorates due to discharge, resulting in production of abnormal images. Further, a low surface energizer applicator enlarges an image forming unit and lowers layout flexibility, and the cost of the image forming unit increases. The low surface energizer is effectively included in a layer to improve the releasability thereof as well.
Japanese published unexamined application No. 2007-178815 discloses a photoreceptor including a fluorine-substituted polysiloxane resin in its surface layer to impart high releasability to the surface thereof. However, a siloxane bond is known to cause polarization and hydrogen bond. Therefore, the siloxane bond adheres to a toner more and the reliability deteriorates under high humidity. Further, in order to expose a low surface energizer on the surface, the surface of a photoreceptor is constantly abraded, and therefore mechanical durability thereof is sacrificed.
Japanese published unexamined application No. 2002-006526 discloses a photoreceptor including a lubricating particulate material in its protection layer.
Japanese published unexamined application No. 2008-139824 discloses a photoreceptor having a surface protection layer formed of a hardened fluorine-containing light curing composition including a (meth)acylate including a fluorinated alkyl group and a photopolymerization initiator.
Japanese published unexamined application No. 2008-233893 discloses a photoreceptor having a crosslinked surface layer formed by hardening a fluorine UV curing hard coat agent and a monofunctional radical polymerizable compound having a charge transport structure, and further including a lubricating particulate material therein. The fluorine material effectively decreases adherence between a photoreceptor and a toner. Particularly when included in a hardened protection layer, the photoreceptor has high mechanical durability and lower adherence to a toner. However, the protection layer needs to include a large amount of the fluorine materials to sufficiently decrease the adherence. The fluorine materials having no charge transport structure increases a bright space potential when included in a large amount. It is thought this is due to hindrance of charge transport in a layer or in an interface between a charge transport layer and a protection layer. Further, a releasing material tends to decrease film strength. Namely, the releasing material is preferably present only at the surface of a photoreceptor.
As a method of making the releasing material present only at the surface of a photoreceptor, Japanese published unexamined application No. 2005-037562 discloses preparing a photoreceptor with a coating liquid including a large amount of fluorine-containing particulate resins. This imparts releasability to the surface of a photoreceptor, but the surface needs to be abraded such that the fluorine-containing particulate resins are exposed. Therefore, mechanical durability of the photoreceptor is sacrificed.
Japanese published unexamined application No. 2006-267859 discloses a method of forming a hardened surface layer, and then burying a particulate material therein. This can bury the particulate material only at the surface of a photoreceptor. However, a circumference of the particulate material buried does not have a structure engulfing the particulate material. Therefore, the particulate material has a small retaining force and is quickly released, resulting in discontinuation of releasability.
Japanese published unexamined application No. 2009-145480 discloses a method of migrating a lubricating filler at the surface of a surface layer after coated. The filler is engulfed by a hardened protection layer. However, the hardened protection layer has small crosslink density, and therefore has a low mechanical durability. Namely, even the lubricating filler is difficult to maintain releasability.
Japanese published unexamined application No. 2002-357914 discloses a method of coating plural filler dispersions to include 2 fillers in a protection layer with a concentration gradient for each in a direction of thickness. This can possibly make the fillers having different properties from each other develop each of their properties. However, most of the fillers are coated by the protection layer. Therefore, even when a filler having releasability is included, releasability is not fully exerted occasionally.
Japanese published unexamined application No. 2001-2323954 discloses a protection layer, from the surface of which at least two fillers project. A filler having a particle diameter larger than a thickness of the protection layer is included in a coating liquid to form the filler projected from the surface thereof. Such a coating liquid tends to form a nonuniform film. Further, the thickness of a protection layer limits a particle diameter of the filler. Since the filler projects while coating, a large area of the filler is coated with resins in the coating liquid and does not sufficiently exert its effect. In addition, the projection of the filler is difficult to control. Japanese published unexamined application No. 2001-235887 discloses a filler projecting from the outermost surface. However, the filler is not fixed well and an amount thereof is too small to exert its effect for long periods.
Therefore, releasing materials need to be gathered near the surface, exposed and kept as they are. However, arts maintaining such conditions for long periods have not been established.
As just described, photoreceptors are difficult to have both high mechanical durability and high releasability, and can have only one of them now.
Because of these reasons, a need exists for an electrophotographic photoreceptor having both high mechanical durability and high releasability, and stably producing high-quality images for long periods even after repeatedly used for long periods.